The alloreactive T cell repertoire has been analyzed for responses to two categories of alloantigens: mutant Kb determinants and non MHC-encoded Mlsc antigens. It was demonstrated by limiting dilution techniques and slope analysis that proliferating F1 T cell populations contain distinct subsets capable of recognizing Mlsc encoded determinants in the context of parental MHC determinants. These findings demonstrate that MHC Mlsc determinants are recognized by responding T cells in association with MHC encoded determinants. T cell clones specific for Mls determinants have been generated, and the MHC restriction of recognition by these clones is being evaluated. Responses to Kb mutant determinants were evaluated employing radiation bone marrow chimeras, neonatal tolerization, and cold target inhibition in assays of cell mediated lympholysis (CML). The results of such studies demonstrated that the generation of the T cell repertoire to these mutant MHC determinants was not the result of T cell genotype alone or of maturation environment alone, but rather represented the outcome of unique interactions between these two variables. This T cell repertoire appears to reflect, at least in part, the activation requirements of cytotoxic T cell precursors, and to be regulated by genes in the K region of the MHC.